Why avoid ECB?

• A. It increases encryption speed
• B. It does not provide semantic security and reveals patterns
• C. It requires an IV
• D. It provides integrity and confidentiality

Answer: (B)

The essential idea is that how a mode processes data determines whether patterns in the plaintext remain visible in the ciphertext. In ECB, each fixed-size block is encrypted independently with the same key. That makes the scheme deterministic: the same plaintext block always turns into the same ciphertext block. Because of this, any repetition or structure in the plaintext—such as repeated words, phrases, or the outlines of an image—shows up as repeated patterns in the ciphertext.

This is why it fails semantic security, which means an attacker should not be able to glean information about the plaintext from the ciphertext beyond its length. Since identical blocks produce identical ciphertext, an observer can detect relationships between blocks and distinguish between different messages, defeating semantic security. The lack of any chaining or diffusion across blocks means there’s no protection against pattern analysis, even though each individual block is encrypted.

ECB does not provide integrity or authentication either; it only offers a basic level of confidentiality for isolated blocks, not for the message as a whole. It also doesn’t depend on an IV, which is why that option is not the core issue here.

Because of these weaknesses, ECB should be avoided. Use modes that provide diffusion and, ideally, authenticated encryption (for example, CBC, GCM, or other schemes that offer both confidentiality and integrity).